Is the weight of water that you've already drunk that much different from the weight of water being carried in your pack?

I ask because I know it's a common practice for people to chug filtered water at a water source, then carry very little water away from the source because their bellies are already full of water. The rationale here is that you don't have to carry as much water weight.

But I'm beginning to question how much different the two scenarios stated in the question above actually are.

Water that is in your digestive tract is probably closer to your center of mass than water carried in your pack, but I would bet this particular difference is rather small in practice.

So that leaves, what? I can't think of anything else…

If you took two identical people, and added 2 pounds of fat to one of them, then I think we would all agree that moving the mass of the fatter person would take more energy. I don't see why we don't apply the same logic to water that has been drunk. That mass certainly doesn't go away until we pee/poop/perspire/respire it out, so we shouldn't treat it like it's massless once it enters our bodies.

This line of reasoning, if sound, makes me begin to question the practice of chugging water at a water source, especially if there is a very long distance to the next water source and your hyper-hydrated bowels just get rid of the excess water in a few hours anyway (via the kidneys… you know… peeing), leaving you thirsty later on in the day when you really need the water (that you could have had if it was in your water bottle instead of in your pee puddle 10 miles back.

Is this making sense to anyone? Am I totally off base here?

I understand that the water in your digestive tract is actually doing something, but I want to restrict this discussion simply to the water weight itself, since you could presumably always just sip water out of your water bottle to get it into your digestive tract and do the same thing as the big water bolus would do (i.e. hydrate you).

I think you basically have it right, but it may be worth considering how weight is carried separately from questions as to how your body can use water, and looking at the issues under a range of conditions.

At one extreme, if I’m walking along a stream and can drink whenever I want, it doesn’t make sense to carry any water, or to drink any more than what I need to stay refreshed and well hydrated. My hydration opportunities are already optimized, and anything I carry in my stomach or on my back is a waste.

At the other extreme, if I have 24 hours to go to my next water source, I might hope to drink 3-4L of water (YMMV) instead of carrying it. There are reasons why I might prefer to carry water in me rather than on my back (see below), but in this particular scenario, the strategy is going to fail miserably (even if I plan things so I won’t be walking too vigorously when my stomach is really full). Not having been made aware of my plan and the dry times ahead, my kidneys will do what they naturally do when faced with a lot of water – get rid of it. I’ll end my 24 hours feeling very parched. Much of that water was wasted.

In between, there may be some situations in which some waste of what looks to your kidneys like excess water is worth it if you get a compensatory advantage from using your body in place of a water bottle. To a first approximation, weight on your torso is weight on your torso (as opposed to weight on your feet), but all other things being equal, it might be slightly better to carry it in your body than on your back. It’s one less thing tugging on your shoulders, it’s one less water bottle to carry, and it’s one less thing adding to what your pack needs to be engineered to carry. These are all minor considerations, in my opinion, since your body can’t productively use enough chugged water to make much of a difference, but if you’re paying top dollar to shave grams from your gear list, it might well matter more than whether you treated yourself to the cuben or settled for sil.

The only time I really think about this stuff in the real world is when I’m heading out for 2-3 hours under circumstances when I can safely skip carrying anything at all if I skip the water bottle. A little water will go to waste, and I’ll be thirsty at the end, but I like the feeling of walking without anything on my back at all.

Water weight is weight no matter where you carry it, but how and when and where you carry it is and should be circumstantial.
Where I live, in the desert, I both tank up and carry, cause not doing so can put you in a bad situation. One never knows if the next water "supply" could be contaminated or spoiled by accident or simply not there. Windmills and cattle tanks are often turned off and dirt tanks can have dead animals or be mud. We don't have nice streams (OK. We have one, but it flows out of Mexico through a mining area) in every canyon or mountain side to rely on. So I drink as much as I can hold and fill my containers and do all my "water chores" at sources. I know how much I use in a 24 hour time period on a hot day and try to make sure I don't "go thirsty". You shouldn't take electrolytes while dehydrated and I take them, so it's important to stay hydrated and not deny your body water cause you just didn't want to carry some weight. I'd rather carry water than feel like doo doo while making miles here. Carrying also gives me more options for camp sites. The only exception to the above is if I am able to do water caching and I've not done that yet but may in the future.

"Is the weight of water that you've already drunk that much different from the weight of water being carried in your pack?"

It can be very different from a comfort perspective. Weight in your belly doesn't pull down on your pack straps and waist belt.

As far as work done hiking it probably is the same or very close to the same where ever the weight is. On the other hand you might save weight on bottles and a possibly use a smaller pack because you have fewer bottles. It might not amount to a lot, but still it can be a weight savings.

But utility wise- water in your pack is doing nothing for you, presently. Water in your body is actively cooling you, helping you digest food, and assisting in providing blood volume and energy and all the other countless cool things water does in our bodies. I know when I'm adequately hydrated, my energy levels are far increased. So from that perspective, the water in your body is able to do its job immediately.

The goal either way is be be well hydrated. If there's lots of water around, like hiking parallel to a river, I don't need to carry much. I wouldn't hike away from water without at least a liter and I'm not adverse to starting out with two on a hot day.

In a similar way, you can top off at the trailhead too.

Adding the new Sawyer mini to my day hiking kit has helped with my water strategy. Chemicals take too long and there is some expense. I still carry a few tablets for backup, but with the Sawyer it's just a nice break to stop and scoop up another liter and also have a drink right there.

There is some mind game that makes me thirsty if I'm low on water– all in the mind of course.

"2 pounds in your stomach is much more tolerable than 2 pounds on your back/shoulders."
My thought as well.
The beauty of having lightweight gear is that I don't notice a few pounds either way, though, and rarely carry more than 1.5L extra beyond my usual 2L(and often carry that 2L when I could technically get away with carrying none, just to save the hassle of stopping and filtering).

I find that water weight is more significant with a minimalist day pack, like a Flash 18 or other "stuff sack with shoulder straps". I find that class of pack has an upper weight threshold that will punish you when you cross it. The weight of another liter can make the straps collapse and the whole thing ride poorly.

Water is a significant portion of pack weight with a small load. I've tried to cook up schemes using a fanny pack or hauling a water container bandolier style to get the weight out of the pack. The fanny pack weight can easily exceed the "stuff sack" pack weight. A bandolier arrangement adds straps, complexity and can bounce around in way of your arms, but it's handy for a drink.

In this example I tied up a simple net with some spare line. It adds 1.5oz in this form. You could do the same with any water container. You could use much lighter line, like braided mason's line. A webbing strap with snap hooks would be good. I rigged the red line until I find a strap on the cheap. There are all kinds of bottle carriers available with straps or Velcro mounts.

As far as the mass you have to move down the trail, no difference. But there are so many other factors – how far apart are the water sources, how reliable are they, how hydrated are you, what have you eaten recently, what is the weather like, and more, that have an effect, that the answer to whether you're better off with a full bottle or a full belly is different is different situations. Though personally, I can't think of a situation where I would go one or the other. If I think I need a full bottle (or bottles) in my pack I am also going to drink up before I leave the water source. But that's just me.

Haven't seen this variable, so I'll add it to a maximizing-the-carry equation: your water's time-value. How long is it good for? Not an SUL, I don't worry about this too much, and do often camel up when I refill bottles. But it is then not long before I racehorse out. People are aware of how much unused water they bring into camp, but less so of how much overload they quickly eject. There is also the intangible pleasure of a quenching drink.

"You are NOT a camel :)"*
When I stated that a few years ago at White Blaze I was called an idiot …
My challenge there was very simple :
have breakfast, drink 3 liters of water , then walk around the block with your pack on and see how long you hold on to those 3 liters and how long you can walk without having another drink.

Next morning repeat without drinking the extra 3 liters of water…
You will find that the first morning ,at the start,you have carried about 3 extra kilos with you.
* from Wiki :

Camels have a series of physiological adaptations that allow them to withstand long periods of time without any external source of water.[17] Unlike other mammals, their red blood cells are oval rather than circular in shape. This facilitates the flow of red blood cells during dehydration[21] and makes them better at withstanding high osmotic variation without rupturing when drinking large amounts of water: a 600 kg (1,300 lb) camel can drink 200 L (53 US gal) of water in three minutes.[22][23]

My wife worked with a remote Aboriginal community for close to a year.
(Ngunpan,WA, pop about 30)
She was the only "white" person there.
The guys often enough went walkabout. That is heading somewhere for days or weeks.
She never saw them "cameling up" not even having an extra glass of water for that matter. in fact they usually did not carry water.
Me or you would die there in a few days but they can find water, maybe not today but probably tomorrow…

I have no problem with the idea of hydrating heavily at a water source, I just think everyone needs to understand that most of that water is going to be peed out in the next two hours and you still should be carrying enough water for future thirst hitting you before the next water source.

I think the other point I was getting at is that chugging filtered water at a water source does have it's downsides, since you are carrying extra water weight in your body. I hear a lot of people talk about this practice of chugging water at a water source and they act as if that water weight doesn't exist anymore because it's not in their pack, which is kind of silly.

On a side note, I watched a documentary on camels a few years ago and I came away from it in awe of their physiology. The various adaptations they possess for dealing with long waterless periods are just incredible.

The phrase "camel up" has always seemed stupid to me, and the act of such seems akin to to stuffing your pockets with things you don't want to count towards baseweight. There's no reason not to drink your fill at a water source. But if you think drinking extra is going to make your body save that water for when you need it later, I have some news for you…

my theory is that you ideally show up to a water source with empty bottles.. meaning you timed it right and didn't carry more than you needed. now you can drink you "next drink" from that empty bottle while at the source. not liters or anything crazy.. but how much you'd drink at a time. if you timed it wrong and have been out for a while this may be more than usual.

now you have a full amount to carry to the next source maximizing the use of the source you are at.

i think the term "cameling up" gets used and misused too much. some people that say it think it means like Franco says and think they can "store" water. others may use it to mean what I do and just drink what you want there and then fill your bottles.

The end of my taking this type of scatological analysis seriously (ok, I never did) was when Ryan Jordan, about 10 years ago I think, in an interview somewhere argued that if you eat the right food you can cut down on toilet paper carried. I took him to mean at the time something like… ahem… his excrement would have the optimal consistency, rather than constipation, which I suppose would also allow cutting down TP weight.

Anyway, that statement was the hardest core UL one I every heard, then or since. Even though I was more serious at that time about it, I realized I would always be a pathetic amateur, and as such, shouldn't pay any attention.

"i think the term "cameling up" gets used and misused too much. some people that say it think it means like Franco says and think they can "store" water"

I have never seen anyone pretending that we can store up water exactly like camels , as in for days.
If you re-read my comments it should be pretty obvious that I am aware people talk about storing up water for hours.
My point is that if drinking 3 liters or so at once (not an extra pint…) you are just carrying inside you extra weight that is of no use at all, in fact you will end up flashing out salts that your body needs anyway.
There is a balance, but in my opinion the more you get used to drink in one setting the more you will need to drink, just like eating…
To me "tto much is always too much" .
How do I know 3 liters is too much ?
That is another story.

I know you didn't mean for days. I meant that people who use the term can mean different things. some people use it as slang for drinking water at a water source for a drink and nothing more. we can find other slang terms that get thrown around that aren't taken literally unless Bob is your uncle?

i don't drink a lot in general so perhaps i've become used to it. being a cyclist i can only carry 2 x 24oz bottles at a time. I tend to only backpack with 1L unless i know the next source is far enough that i need 2.

i do try to extend that capacity by drinking a bit before i leave, a bit at sources and then rehydrate at camp (or after my rides)

Keep in mind that the average body can only absorb about 1.0 liter per hour from the stomach. The rest just sits there, doing whatever unabsorbed water does. I would rather carry my extra water in a Platy, outside of my body, and then sip it later as needed. But YWCHMV–your water consumption habits may vary.

Shows that the half life of water absorption is 11-13min. So if you drink 1L at your rest stop, in about an hour you've more or less absorbed most of it. That being said, 'storing' water in your stomach isn't going to work but it will be stored in your bloodstream unless you overhydrate and piss it out. On the other hand, if you sip your water throughout the day, you're limiting the amount of water absorbed (1 sip = 10ml? so you only absorb 5ml in 11 minutes… kind of a waste of effort). I would (and have!) calculate the amount of water you need per mile or per hour on a given terrain and drink your fill using this 10min half life measurement. It works great on the PCT between water sources… for me it's 1L drunk in one go, walk 4 miles, drink 1L again, and 4 miles later I hit a stream… repeat